Most folks fix this problem by one or a combination of the following three methods:
1. Keep your rpm up around 2000 when working your tractor. If you lug down the engine, the hydro pedals tend to not return to neutral.
2. Adjust the return spring for a quicker return. New Holland has increased the spring size and you should be able to adjust it by looking up under the right side of the operator's platform (Item 16 in the attachment).
3. Spray lube the roller on the actuator arm (item 26) and the bottom surface of the plate that looks like a "B" laying on its side (yellow highlight).